A Close Encounter with Asteroid Eros

NASA's NEAR Shoemaker spacecraft swooped 3 miles
above the surface of 433 Eros on Oct 26th, marking its closest-ever
approach to the tumbling space rock.

October 26, 2000 -- Early
this morning, NASA's NEAR Shoemaker spacecraft, which has been
in orbit around asteroid 433 Eros since February, swooped just
3 miles (about 5 km) over the tumbling space rock. The elevation
of the flyby was similar to the cruising altitude of a commuter
jet on Earth. No space probe has ever been so close to a minor
planet.

"Although NEAR was very close to Eros -- the closest
we've been before was about 35 km in July -- the spacecraft was
never in any danger," says Andrew Cheng, the NEAR
Shoemaker project scientist at Johns Hopkins University.
"We chose to fly over an area of the southern hemisphere
where, if we were off-target, the uneven gravity of the irregular
asteroid would actually kick us back into a higher orbit."
Compared to a commercial airliner flying hundreds of miles per
hour above Earth, NEAR traveled slowly through Eros's weak gravitational
field. Its maximum speed was only 14 miles per hour (23 kph).

Above: This image was taken in the early hours of October
26, 2000 as NEAR Shoemaker was skimming over the surface of Eros.
Most of the 350-meter wide scene is covered in rocks of all sizes
and shapes. The large boulder just below the center of the picture
is about 15 meters (50 feet) wide. The smallest visible rocks
are about 1.4 meters (5 feet) across. [more
information]

The Johns Hopkins University Applied Physics Lab, which manages
the NEAR mission for NASA, announced
on Wednesday that the flyover had gone as planned and that the
spacecraft was heading back to a higher orbit.

While the dangers from skimming so close to Eros were slight,
the potential rewards were great.

"One of the mysteries we've encountered on Eros
is a curious deficiency of small craters," explained Cheng.
"Something seems to be obliterating impact features smaller
than a few tens of meters across."

On worlds that are peppered with impact scars (like the Moon
or Mercury) there are always many small craters for each large
one. That's true on Eros, too, but images of the asteroid collected
during the first 8 months of the NEAR mission reveal fewer small
craters than researchers expected. On Earth small impact scars
wear away because of weather, but there is no weather on airless
Eros. Some other process must be at work and scientists would
like to know what it is.

"The high-resolution pictures we captured today will
show these small scales very clearly," says Cheng. "They
may give us some hints about what's going on."

While Eros seems to be running low on diminutive craters,
it boasts a surprising surplus of boulders.

"Clark Chapman of the Southwest Research Institute has
noticed that the surface of Eros is littered with 10- to 20-meter
wide boulders, many more than we would expect [by simply extrapolating
the number of large boulders to smaller sizes]," continued
Cheng. "This is telling us that there's something funny
about Eros's cratering history in the 'recent' geological past.

Above: Another image from NEAR Shoemaker's Oct. 26 low-altitude
flyover of Eros. The large boulder near the bottom of the image
is about 25 meters (82 feet) across. [more
information]

"One possibility is that the cratering rate plummeted
a billion or so years ago when Eros exited the main asteroid
belt between Mars and Jupiter to become a near-Earth asteroid.
After that, there may have been too few impacts to pummel these
boulders into smaller pieces. These close-ups of Eros may tell
us if the overabundance of 10-meter rocks extends to smaller
sizes as well -- that would be an important clue."

Four months from now, NEAR Shoemaker will be poised to record
an even closer view of Eros. "We're considering landing
on the asteroid at the end of NEAR's one-year mission,"
says Cheng. "The spacecraft would touch down near the south
pole of Eros where the rotational surface velocity is low."

Fans of Arthur C. Clark's science fiction novel "Rendezvous
with Rama" might recall that explorers in that story
landed near the pole of an asteroid-sized cylindrical spaceship,
a spinning behemoth about the same size and shape as Eros. They
chose to touch down near Rama's spin axis for the same
reason that NEAR would settle near Eros's south pole; it's easier
to land where the ground is moving slowly.

Mission
scientists are still reviewing various end-of-mission scenarios
and expect a final decision on whether the spacecraft will land
and how by the first week of December.

"NEAR was designed to orbit Eros, not to land on it,"
says Cheng. "Most of the science instruments won't even
work so close to the asteroid's surface. We want to do this as
a proof of concept, to show that a spacecraft can land on an
asteroid." Future missions to explore and possibly return
samples from the minor planets will depend on maneuvers that
NEAR might soon try for the first time.

"Things could go wrong," Cheng stressed, like crashing
into one of Eros's many boulders. But if NEAR touches down without
mishap and can still communicate with Earth, scientists will
enjoy a brief close-up of Eros that will make today's flyby seem
remote by comparison.

For more information about asteroid Eros and the NEAR mission,
please visit the Near-Earth Asteroid Rendezvous mission home
page at http://near.jhuapl.edu.
The Johns Hopkins University Applied Physics Laboratory in Laurel,
MD, designed and built the NEAR spacecraft and manages the mission
for NASA.

Square
Craters
-- NASA's NEAR Shoemaker spacecraft has spotted square-shaped
craters on asteroid Eros, a telltale sign of mysterious goings-on
in the asteroid belt long ago.

Asteroids
Have Seasons, Too
- Later this week, the Sun will rise over the south pole of asteroid
Eros, revealing unexplored terrain to the instruments on NASA's
NEAR-Shoemaker spacecraft.

Â Facts
about Eros

Eros
circles the Sun once every 1.76 Earth years. It spins on its
axis once every 5.27 hours.[more]

Eros is about 21 by 8 by 8 miles
(33 by 13 by 13 kilometers) in size. Its shape has been compared
to a shoe, a battered boat, or a peanut. [more]

The gravity on Eros is very weak
but enough to hold a spacecraft. A 100-pound (45-kilogram) object
on Earth would weigh about 1 ounce on Eros. [more]

Eros is "Near-Earth Asteroid"
or NEA. Its next close approach to Earth will come in January
2012, when it will pass 0.178 AU from our planet. Although Eros
is a NEA, there is no chance that it will
collide with Earth.[More]

Eros
or Bust
- February 8, 2000. SpaceScience.com. NASA's Near-Earth Asteroid
Rendezvous Mission is nearing 433 Eros. It is scheduled to go
into orbit around the space rock on Valentines Day, 2000.